System for regulating web tension



A g- 6, 1969 P. DE HERTEL EASTCOTT 63,

SYSTEM FOR REGULATING WEB TENSION Filed April 22, 1968 2 Sheets-Shes 1 N mik MQOMZQU WEE 3 5.10

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/;.\'"/0R. PETEP DE HERTEL EASTCOTT Wei/ am? FATE/viva Aug. 26, 1969 P. DE HERTEL EASTCOTT 3,463,414

SYSTEM FOR REGULATING WEB TENSION 2 Sheets-Sheet 2 Filed April 22, 1968 RHEOSTAT 55 CONTROLS MOTORS M1 AND M2 lz.\"'l()/\.

PATENT AGENT United States Patent 3,463,414 SYSTEM FOR REGULATING WEB TENSION Peter de Hertel Eastcott, Peterborough, Ontario, Canada,

assignor to Canadian General Electric Company Limited, Toronto, Ontario, Canada, a corporation of Canada Filed Apr. 22, 1968, Ser. No. 722,873 Claims priority, application Canada, Apr. 29, 1967, 989,211 Int. Cl. B65h 25/22 US. Cl. 242-75.44 9 Claims ABSTRACT OF THE DISCLOSURE A closed loop system for regulating the tension in a moving web of paper compares an electrical reference signal representing a particular web tension with an electric signal representing a measure of tension to obtain a tension control signal. The measured signal is obtained by drawing a loop in the web with constant Suction and beaming light onto its outer curved surface at an angle that reflects light from the surface according to curvature onto an array of solar cells which generate an electric signal representing web tension. The control signal is modified by a third electric signal obtained from a program synchronized with the web handling equipment. This third signal is varied according to the diameter of the roll of web being wound, and as a result, web tension now varies with roll diameter.

This invention relates to a system for regulating the tension in a web of a pliable material such as paper according to a preset program While the web is advanced from one station to another at a high speed.

The applicants United States application, Ser. No. 703,222 filed Jan. 22, 1968 describes a closed loop feedback regulating system for controlling the tension in a web of pliable material such as paper while it is advanced at high speed for one roll to another. This system uses a reference signal which can be set manually or otherwise to represent the web tension wanted, and a feedback signal which represents the web tension actually measured by a tension measuring device located somewhere between the rolls. These two signals are summed algebraically to obtain a control signal which is used to control web tension, as for example, by controlling the braking force applied to the roll being unwound. This system is designed to maintain web tension relatively constant at the value set by the reference. Even though uniform tension is maintained in the web as it advances, it is difficult to wind a roll in which the layers are at a uniform tension throughout the roll because there are certain variables inherent in the web handling system. For instance, during starting of a roll, the turns tend to be too loose because the roll is still so small and light that it lags behind the driving rolls in accelerating, but near the finish of the roll during slow down, the turns tend to be too tight because the inertia of the roll is now so great that the roll fails to decelerate with the driving rollers, that is, the roll tends to continue rotating at the same speed even though the driving rollers are decelerating. Another variable to contend with is the effect that the driving rollers have on tension as the roll increases in weight; the heavier the roll, the more the rollers tend to disturb the layer of the outer turns. These are some of the variables which influence layer tension.

Although the mechanics of these variables are not too well known, methods of compensating for their influence on layer tension are known. One of the better known methods of compensation is to vary web tension according to a specific program rather tha holding it constant throughout the winding of a roll. Prior art means used in carrying out this method are essentially mechanical de- 3,463,414 Patented Aug. 26, 1969 ice vices subject to the disadvantages and limitations usually associated with mechanical devices in this art. These mechanical devices tend to get in the way because they are usually large and mounted directly on the apparatus; they also tend to be slow in response time and subject to errors. As a result, they are not entirely satisfactory, particularly with the modern high speed web handling equipment now in use.

It is, therefore, an object of this invention to improve on the compensation outlined in the foregoing paragraph.

According to the invention, Web tension is varied according to a program which takes into account variables such as acceleration, deceleration and diameter of the roll being wound. The program is designed to control web tension such that the residual tension in the layers throughout the roll follows a predetermined pattern from the beginning to the end of the roll. This is done by means of light beamed onto a light activated device and means responsive to the diameter of the roll for varying the light falling on the device according to the program. The device gives an electric signal varying in magnitude with the light falling on it. This signal is then applied in the closed loop regulating system for modifying the reference signal, which, in turn, causes web tension to follow the program as the roll increases in diameter.

The following detailed specification will aid in a fuller understanding of this invention when taken in conjunction with the appended drawings i which:

FIGURE 1 is a diagram of a regulating system embodying the invention;

FIGURE 2 is a cross section of the programming device;

FIGURE 3 is a plan view of the disc used in the programming device;

FIGURE 4 is a diagram of the electric circuits used for a stepped speed reduction in the programming device; and

FIGURE 5 is a plan view of another disc.

A closed loop regulating system will now be described with reference to FIGURE 1. In this figure there is shown a system for regulating the tension in a web 10 of a pliable material such as paper released from a roll 11 and advanced at high speed over a tension detecting assembly 13 in the direction 12 onto a roll 14. An electrical signal obtained in assembly 13 representing web tension is fed via line 15 to a summing amplifier 16 where it is summed algebraically with an electrical signal set by a rheostat 17. The signal set by the rheostat, in this instance manually by the operator, is a reference representing a particular web tension, and the signal from the tensio detecting assembly is a feedback signal which initiates any corrective action necessary to hold web tension within the tolerances of the value set by the rheostat. After summation and amplification at 16, the combined, or what will hereinafter be called the control signal, is further amplified in power amplifier 18 and thereafter applied to control the tension in the web as it advances from roll 11 to roll 14 in a way to be described later. Another electrical signal also representing web tension and obtained in assembly 13 is fed via line 19 to a meter 20 which is calibrated to read web tension directly in pounds per lineal inch of Web width. This meter tells the operator what the measured value of web tension actually is at any particular moment, and it is also used by the operator for calibration purposes to be dealt with later.

Assembly 13 consists essentially of a pedestal 21 mounted on a foundation and supporting a flat, horizontal base plate 22 on which there is mounted a draw table 23 directly above the pedestal and a guard board '24 spaced to the left of the table. The table has a wall 25 rising vertically from the base plate and then curving to the right at 26; the guard board has a similar wall 27 spaced to the left of wall and also rising vertically from the base plate but to a lower point 28 where it makes a right angle turn to the left and then curves downwardly at 29. An end wall located at each end of the draw table and guard board close the ends of the channel defined by walls 25, 27, and plate 22; neither of these end walls have been shown in FIGURE 1 in the interest of drawing clarity. The end walls may be simple plugs which rest in the channel at the edges of the loop and which are adjustable so the assembly can handle webs of different widths. These plugs are low enough that the loop can pass freely over them Without being obstructed by them, thereby allowing for variations in web width and lateral movement. Another type of end wall well suited for use in assembly 13 is described in the applicants United States application, Ser. No. 699,604 filed Jan. 22, 1968. The end walls and the channel define a box-like structure disposed transversely to web 10 and having an open top side and a cavity 30.

The guard board and draw table are adapted to allow Web 10 to be drawn freely over them as illustrated with a shallow loop such as that indicated at 31 or 32 drawn into cavity 30 by means of suction in the box at a pressure a little below atmospheric pressure. Vacuum may be applied to the box i any known Way, as for example, by means of an exhaust fan at each end of the box having its intake in communication with cavity 30 near the end walls; one of these fans is illustrated at 33. The end walls are located near the edges of the web so as to minimize the amount of air drawn into the box at the ends of the loop. Curved surfaces 29 and 26 guide the web into and out of the loop and maintain it in the proper position between the end walls of the box.

For any given pressure of the suction in the box, the depth of the loop is an indicatio of the tension in the web; hence, by holding the pressure reasonably constant a signal representing the tension in the web can be obtained. Loop depth is defined as the amount that the web deflects from a plane surface tangential with respect to the curved surfaces 29 and 26. Fans 33 are a type which can be set to maintain a relatively constant negative air pressure in the box even though the quantity of air admitted to the box varies considerably. The expression relatively low negative air pressure used in the specification and claims is intended to mean a vacuum or suction at an air pressure a little below atmospheric pressure.

In the embodiment of the invention illustrated in FIG- URE 1, a beam of light 34 from lamps 35 is directed onto the under surface of the loop at an acute angle thereto. Some of this light is reflected as indicated at 36 down onto a light responsive device 37 located below the reflecting surface; the amount of light reflected will depend o the curvature or depth of the loop, the deeper the loop the less light falling on device 37. Lamps 35 are enclosed in an opaque box 38 supported on base plate 22 and having a transparent window 39, i.e., glass window, in its top side for light 34 to pass through from the lamps to the under surface of the loop. Device 37 is enclosed in an opaque box 40 supported on box 38 and having a transparent window 41, i.e., glass window, in its top side for admitting the light 36 reflected from the under surface of the loop. To facilitate the setting of device 37 for full exposure to the brigh area on the loop illuminated by beam 34, both device 3-7 and its enclosure 40 are adjustable up or down. Device 37 may consist of a number of solar cells that generate electrical signal energy proportional to the bright area. As this signal level is very low and readily affected by outside interference, shielded conductors are used at 15 and 19 to connect the cells to amplifier 16 and meter 20. The cells are connected in such a way that the signal energy is divided in a set proportion between the amplifier and meter.

The electrical signal from device 37, representing web tension as measured in tension detecting assembly 13, is summed algebraically with a reference signal of the same character from rheostat 17 in the summing amplifier 16, the reference signal representing the tension that the 4 system is set for. In the system illustrated, the signal from device 37 is DC of varying amplitude and the reference signal is DC. A set proportion of the signal from device 37 is applied to meter 20 so that it continually provides a reading of web tension in pounds per lineal inch of web width. The signal energy for rheostat 17 may come from a direct current source, or some other known source such as a constant voltage transformer with an adjustable output. After summation and amplification i amplifier 16, the combined signal is fed via line 42 to power amplifier 18 where it is amplified sufficiently to render it useful for web tension control purposes. In the system illustrated the output from amplifier 18 is fed via line 43 to an auxiliary field winding 44 of a booster generator G2 in the electrodynamic braking system 45 for the roll of paper 11 being unwound. Amplifiers 16 and 18 are both well known types.

As web 10 is withdrawn from roll 11, it passes under a roller 46, over tension detecting assembly 13, through a slitter 47, over a roller 48, under rollers 49 and onto roll 14. Rollers 46 and 48 guide the web over the tension detecting assembly and through the slitter where the edges of the web are trimmed and the web is cut into a number of strips. Actually, number 14 represents a number of rolls of web, one for each strip. These rolls rest on and are driven by rollers 49 and 50, the rollers which advance the web and wind it up into roll 14. Rollers 49 and 50 are driven by motors M1 and M2 respectively.

In advancing the web, motors M1 and M2 do so against braking torque applied to roll 11 by means of the electrodynamic brake 45. By utilizing the signal output from amplifier 18 in the electrodynamic brake to control the braking torque, it is possible to hold web tension relatively constant Within predetermined limits. The electrodynamic braking system illustrated at 45 is a well known system wherein a generator G1 driven by roll 11 is connected in parallel with a main generator G3 and the power output therefrom supplied to motors M1 and M2. In such a system, the energy produced by braking is used to power the driving motors, and not simply wasted. The torque which generator G1 applies to roll 11 is controlled by means of a booster generator G2 driven by the main generator G3 and having its armature connected in series with the armature of generator G1. Generator G2 passes current through generator G1 of a polarity which causes the latter to apply torque to roll 11 at all roll speeds, even at standstill. The magnitude of this torque depends on the current passed, and the current passed depends on the excitation of booster generator G2. Therefore, by applying the output from amplifier 18 to a suitably designed exciting winding 44 in the booster generator, the braking torque applied to roll 11 is, in effect, placed under the control of the regulating system.

In place of an electrodynamic braking system such as 45, it is possible to use a fluid regulator and brake of the type described in United States Patent 3,266,376,, issued Aug. 16, 1966. In this system all the braking energy appears as heat and must be dissipated; it cannot be used in the drive as with the electrodynamic brake.

In addition to rheostat 17 and meter 20, the operators console has many other control devices, of which those essential for the operation of the tension regulating system are: a selector 51, a pushbutton 52, a vacuum gauge 53, and a voltage adjustor 54. Selector 51 is used to select a mode of operation from three positions, namely, regenerative, in which none of the controls in the closed loop feedback system operate, indicate, in which the lamps and fans operate so that an indication of tension is given, and automatic control, in which the fans and lamps operate and a corrective signal is applied in the system to keep web tension constant at a set value. Pushbutton 52 is used to start or stop the fan motors through a line and a magnetic starter and interlocks. Gauge 53 is connected directly via line 56 to the tension detecting assembly and gives a direct reading of the vacuum pressure therein. Adjustor 54 is a means for adjusting the voltage of a constant voltage source, e.g., an adjustable constant voltage transformer, connected to lamps 35 through line 57 for setting the level of illumination from the lamps. Another control shown on the operators console is rheostat 55. This rheostat is used for setting the reference signal which controls the speed of motors M1 and M2. It is also shown in FIGURE 4, and its function will be described later in connection with the programming device.

An arrangement of lamps and light responsive device found to give good results will now be considered briefly. Four 6 volt, 25 watt incandescent lamps 35, connected in parallel, operate at approximately 4.5 volts for long lamp life. They are placed in a row inside enclosure 38. Eight solar cells 37 are placed in a row inside enclosure 40 directly above and to the right of the lamps. Every other one of the solar cells are connected in parallel and to meter 20 via line 19, and the remaining cells are connected in series and to amplifier 16 via line 15. The reason for the two different cell circuits is that this is a convenient way of matching the impedance of the circuit with the impedance of the load. As the lamps produce a significant amount of heat, enclosure 40 is sometimes insulated from enclosure 38 to avoid overheating the cells. The assembly of lamps and cells is supported on base 22 transversely of and in the middle of the web. In this particular scheme, webs of newsprint up to 260 inches in width were advanced at a regular production speed of 7200 f.p.m., the loop span was approximately 12 inches, and the vacuum was held at a pressure of about 1.0 inches of water.

During the normal course of unwinding roll 11, slitting the web at 47 and then rewinding the strips into rolls 14 with selector 51 set for automatic control, the web moves at high speed over tension detecting assembly 13 where a loop is drawn in the web of a depth depending on web tension. Since the vacuum acting on the web to form the loop is held at a constant negative air pressure, and can be readily checked by the operator by looking at gauge 53, the depth of the loop is proportional to tension. Hence the amount of light from the lamps reflected from the underside of the loop onto the solar cells will cause the cells to generate electrical signal energy of a magnitude representing web tension. The cells connected in parallel energize meter 20, causing it to give a reading visible to the operator of web tension in pounds per lineal inch of web width. The cells connected in series provide a signal for amplifier 16 representing the web tension measured. The amplifier compares this signal with a reference signal set by the operator on rheostat 17 to represent the tension he actually wants. After comparison and amplification at 16, the combined signal is amplified in amplifier 18 and then applied to field winding 44 of booster generator G2 for the control of the torque that generator G1 applies to roll 11.

The system described so far is designed to maintain web tension at a relatively constant level regardless of the diameter of roll 14. Since uniform web tension does not necessarily produce a roll having the layer tension wanted, this system is modified in such a way that as roll 14 increases in diameter the web tension varies according to a program developed to give the layer tension wanted, or as near thereto as possible.

A preferred means for varying web tension with roll diameter will now be described with reference to FIG- URE 1 where such means is illustrated schematically at 60. This means employs a source of light 61 beamed onto a light responsive device 62, a movable shutter 63 for intercepting a portion of the light beam according to shutter position, and a linkage 64 for moving the shutter as the center of roll 14 raises due to the roll increasing diameter. Device 62 generates an electric signal proportional in magnitude to the light 65 bypassing the shutter and falling on the device. This signal is divided in a rheostat 66 and a set proportion applied by way of a shielded cable 67 to summing amplifier 16, where it is summed algebraically with the reference and feedback signals so that the output signal from the amplifier is modified accordingly. The shutter is designed such that the light bypassing it varies with the position of the shutter according to a predetermined program; hence the modified control signal also varies according to this program, as does web tension. As a result, the tension in the turns of roll 14 is now determined by the program.

In the assembly shown in FIGURES l to 3, light source 61 is a six volt, 25 watt, heavy filament, incandescent lamp operated from a voltage stabilizer at a constant voltage which is set by an adjustable resistor in series with the lamp at a value of about 4.25 volts for a constant light output over a long period of time. Device 62 is a group of solar cells of the type used at 37. Shutter 63 is a metal disc having a periphery 68 defined by a progressively decreasing radius beginning and ending at step 69 and a hole 70 located where the light beam can pass for one angular position of the disc. The disc is best illustrated in FIGURE 3 where the solid line 68 designates the outer curved edge of the disc and the broken line 71 the circle from which the disc was developed. Hole 70 is located near the periphery of the disc to the left of step 69. The disc is mounted on a shaft 72 along with wheel 73 for rotation by the wheel through the medium of linkage 64 which drives the wheel clockwise as the core shaft elevating mechanism 74 for roll 14 moves up along the vertical guide rails 75 as the roll increases in diameter. For the sake of simplicity, the linkage is illustrated in FIGURE 1 as a cable passing over a pair of pulleys 76, 77 and secured to mechanism 74 and wheel 73. The weight of the mechanism rotates the wheel counterclockwise against the force of spring 78 as the mechanism is lowered, and the spring rotates the wheel clockwise as the mechanism is raised. In actual practice, the wheel will probably be a chain sprocket driven by a chain already in use on the core shaft elevating mechanism of the winder.

Reference should now be made to FIGURE 2 for particulars on the construction of a suitable programming device. It has a base 79 secured inside a case 80 closed by a cover 81. The base has a central bore fitted with a pair of sleeve bearings 82, 83 which support shaft 72 for rotation and restrain it from axial movement. The base also has a pair of recesses 84 and 85 at the back side thereof on opposite sides of the shaft. A large opening 86 passes through the base from its front face 87 into recess 84, and a similar opening 88 passes through the base into recess 85. The outside of opening 86 is covered by a plate 89 having a pair of holes 90 and 91 therein, and the inside of the opening is covered by a plate 92. Disc 63 is located directly in front of plate 89 partially covering hole 90 and completely covering hole 91 except for one position of the disc when holes 91 and 70 are aligned. It is secured to the shaft by means of a nut 93 and locating pin 94 and is readily removed and replaced by a disc of another contour. Lamp 61 is located directly in front of the disc opposite holes 90 and 91 so that light is directed through these holes when not interrupted by the disc. Light responsive device 62 1s mounted on plate 92 inside opening 86 and faces the hole 90 in plate 89. Light from the lamp not obstructed by the disc can therefore reach the device through this hole. The amount of light passing through the hole depends on the angular position of the disc, i.e., on the extent that the edge 68 of the disc is set in from c1rcle 71. A Light Activated Silicon Controlled Rectifier (LASCR) 95 located in hole 91 facing the disc is used to slow down driving rolls 49 and 50 some time before roll 14 is fully wound, allowing the outer turns to be wound at a much lower speed. The LASCR is conductive only when the hole in the disc exposes it to the light from the lamp; otherwise it is non-conductive because the disc cuts off the light. The leads from devices 62 and 95 are brought into recesses 34 through the small hole 96 in plate 92, where they are connected to other electrical components as shown in FIGURES 1 and 4.

Light responsive device 62 is connected to a rheostat 66 as shown in FIGURE 1. The rheostat is located in recess 84 where it is readily accessible for adjustment, and it is set manually to select a definite proportion of the signal energy from the device. This signal is applied to summing amplifier 16 by way of shielded cable 67, where it is used for purposes of modifying the reference signal.

A circuit for slowing down the drive exemplified by M1 and M2 in FIGURE 1 is shown in FIGURE 4. In this circuit, the LASCR is connected in series with a Zener diode 97 and a resistor 98, and the series combination connected across the output terminals of the rheostat 55 which sets the voltage of the reference signal that controls the speed of the motors driving roll 14. The input terminals of the rheostat are connected to a source of direct current, and the reference signal is taken from across the LASCR and Zener diode by way of leads 99 and 100. FIGURE 4 shows the components essential to the slow down circuit; in practice other devices will be used in combination with these components in the speed control system. The Zener diode, LASCR and the bias resistor 101 for the LASCR are located inside case 80 in recess 84, while the remaining components are located elsewhere in the system.

In the system mentioned earlier for advancing a web of newsprint 260 inches wide at a regular production speed of 7200 feet per minute, resistors 98 and 101 had 1,500 and 68,000 ohms respectively, Zener diode 97 became conductive at 8.2 volts and the reference voltage was set by rheostat 55 at 40 volts. Since the reference is a voltage only which is compared with a measured voltage representing web speed, negligible current flows in resistor 98, and as a result the reference will remain at 40 volts as long as the LASCR is non-conductive. However, once disc 63 rotates to the position where its hole 70 exposes the LASCR to the light from lamp 61, the LASCR turns conductive, whereupon the Zener diode breaks down at 8.2 volts allowing current to flow in the circuit comprising the elements 98, 95, 97, and 55. As a result the reference voltage decreases to 8.2 volts, the reference voltage representing a web speed of 15 feet per mlnute. To decrease the speed even further, the operator turns the rheostat to a reference voltage lower than 8.2 volts, at which point the Zener diode cease to conduct and the speed is again determined by the rheostat setting. Once the Zener diode ceases to conduct, the LASCR IS reset to its non-conducting state. In practice, once the speed has decreased to 1500 feet per minute, the operator w1ll take over and gradually reduce speed as he completes the winding of roll 14. When the next roll is started, the (1180 is turned back to the roll starting position where the LASCR is again cut off from the light. Hence, in starting a new roll, the operator will wind the first few turns under manual speed control; he will then set the rheostat for a 40 volt reference and let the motor control system take over. The rate that the motors can accelerate is regulated by means of a speed ramp which allows the motor speed to increase linearly with respect to time. The speed ramp regulates deceleration in the same way. Web tension will again be determined by the reference signal, the feedback signal and the programming signal, the programming signal being the signal from light response device 62 that varies according to the angular position of shuter 63.

During the winding of a roll 14, the reference signal remains constant at the value set by rheostat 17 and the signal from device 62 changes as the roll increases diameter. Hence the overall reference signal for controlling tension will now be the algebraic sum of these two signals. The web tension that the feedback signal from device 37 attempts to hold braking system 45 at may be thought of as represented by the algebraic sum of the signals from rheostat 17 and device 62. In summing amplifier 16, however, the three signals are summed algebraically together to obtain the control signal, rather than first summing the two reference signals and then summing the sum thereof with the feedback signal.

After completing a roll 14 and before starting a new one, the web may be drawn taut straight across the guard board and draw table. With the web normally in a loop now a fiat reflecting surface, the light activating the solar cells in the tension detecting assembly gives a reading on meter 20 which can be used to check the lighting level against a standard established for the tension measuring meter in ft. candles/sq. ft. This reading may appear as a red mark on the face of the meter opposite which the pointer of the meter rests when the lighting level is as it should be. If the level is either too high or too low, the pointer will be off the mark, and the level can then be reset by turning the knob on voltage regulator 54 to raise or lower the lamp voltage as necessary. This calibrating means prevents changes in the colour and reflectance of the web from introducing errors in the tension measurements. When satisfied with the lighting level, the operator depresses button 52 to start the fans and thereby restore the vacuum in the tension detecting assembly. He will now return the system to automatic control of tension and observe the vacuum gauge to ensure that the vacuum settles down at the required pressure. Once the system is back on automatic control, the operator will glance periodically at meter 20 and gauge 53 to satisfy himself that the tension and the vacuum are what they should be.

Amplifier 16 may have two stages interconnected through a gain change network. The signals are summed in the first stage, and the resultant signal is fed to the second stage through the gain change network which reduces the gain of the system at web speeds below 1500 feet per minute. The second stage is connected as an integrator. This ensures that sudden large changes in the resultant signal do not immediately cause large changes in the excitation of booster G2, but that a smooth corrective signal is produced of magnitude and character that restores web tension to the set value Without abrupt changes which could cause the web to break or the ends on the rolls being wound up to become uneven. The output of the second stage is amplified in the power amplifier and then applied to a field winding of the booster generator for controlling the braking torque that the braking generator applies to the roll being unwound.

The light control shutter may have more than one peripheral edge such as that shown at 68 in FIGURE 3, as for example, two edges 103 and 104 as illustrated in FIGURE 5 for disc 102. When two edges are used, cavity 88 will be covered by means of plates such as 89 and 92 and a light responsive device such as 62 mounted therein, another lamp such as 61 will be located outside the disc opposite opening 88, and the disc will make somewhat less than one half revolution during the winding of a roll 14. In this case, edge 103 will control the programming signal from device 62, and edge 104 will control the signal from the other light responsive device. This fourth signal may be used to control the speed of the drive for roll 14 so that there is a slight difference in speed between motors M1 and M2. Disc 102 is designed to give a fourth signal representing speed differences that compensate for web tension as roll 14 enlarges.

As those skilled in the art are well aware, a modern paper making operation is far more complex than what has been illustrated and described. The illustrations and description are believed to be adequate for a full understanding of the invention by such a skilled person.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A closed loop feedback regulating system for holding a web of a pliable material such as paper under predetermined tension during advancement of the web from a first to a second station comprising: driving means at said second station for advancing the web; braking means at said first station for applying a braking force to the web resisting advancement thereof and thereby placing the advancing web in tension; a vacuum device located between said stations on one side of the web for holding a shallow loop in the web transversely thereof, said loop being held by means of relatively constant negative air pressure in said device whereby the depth of the loop varies according to web tension; a light source for projecting a beam of light onto the outer curved surface of said loop at an acute angle thereto, said surface reflecting light of an amount varying with said depth and hence with said web tension; means activated by light reflected for generating an electrical signal varying in magnitude with the light reflected and therefore representing the web tension measured; means for providing a reference signal representing a predetermined program of web tension for the regulating system to hold; means for comparing the two signals and obtaining a control signal representing the difference between the measured and set values of web tension; means for amplifying the control signal; and means for applying the amplified control signal to said braking means for controlling said braking force so as to hold web tension within definite limits of the program.

2. A closed loop feedback regulating system for holding a web of a pliable material such as paper under predetermined tension during advancement of the web from a first to a second station comprising: driving means at said second station for advancing the web; braking means at said first station for applying a braking force to the web resisting advancement thereof and thereby placing the advancing web in tension; a vacuum device located between said stations on one side of the web for holding a shallow loop in the web transversely thereof; said loop being held by means of relatively constant negative air pressure in said device whereby the depth of the loop varies according to web tension; at light source for projecting a beam of light onto the outer curved surface of said loop at an acute angle thereto, said surface reflecting light of an amount varying with said depth and hence with said web tension; means activated by light reflected for generating an electrical signal varying in magnitude with the light reflected and therefore representing the web tension measured; means for setting a reference signal representing a constant value of web tension; means activated by light for generating a third electrical signal varying in magnitude according to a predetermined program; means for summing the three signals algebraically to obtain a control signal; means for amplifying the control signal; and means for applying the amplified control signal to said braking means for controlling said braking force so as to hold web tension within definite limits of the level set by the reference signal as modified by the third signal.

3. The system defined in claim 2 wherein said driving means rolls the web into a roll and the means for generating said third signal consists of the following: another source of light of a constant light output; a movable shutter having one side facing the light source so as to block off some of the light and allow some of the light to pass by; a light responsive device located on the other side of said shutter exposed to the light bypassing the shutter; and means for moving said shutter in response to changes in the diameter of said roll for varying the light bypassing the shutter as the roll changes diameter.

4. A closed loop feedback regulating system for holding a web of a pliable material such as paper under predetermined tension during advancement of the web from a first a second station comprising: a pair of driven rollers at said second station for advancing the Web and rolling it up into a roll; braking means at said first station for applying a braking force to the web resisting advancement thereof and thereby placing the advancing web in tension; a vacuum device located between said stations on one side of the web for holding a shallow loop in the web transversely thereof, said loop being held by means of relatively constant negative air pressure in said device whereby the depth of the loop varies according to web tension; a light source for projecting a beam of light onto the outer curved surface of said loop at an acute angle thereto, said surface reflecting light of an amount varying with said depth and hence with said web tension; means activated by light reflected for generating an electrical signal varying in magnitude with the light reflected and therefore representing the web tension measured; means for setting a reference signal representing a constant value of web tension; means following the position of the center of said roll employing light for generating a third electrical signal varying in magnitude according to a predetermined program; means for summing the three signals algebraically to obtain a control signal; means for amplifying the control signal; and means for applying the amplified control signal to said braking means for controlling said braking force so as to hold web tension within definite limits of the level set by the reference signal as modified by the third signal.

5. The system defined in claim 4 wherein the means for generating said third signal comprises another source of light of a constant light output; a disc mounted for rotation with one side facing the light source so as to block off some of the light and allow some of the light to pass by it; a light responsive device located on the other side of the disc exposed to the light bypassing the disc, said disc having its periphery of a contour representing said program; and means for coupling the disc to the mechanism for elevating said roll so vertical movement of the roll rotates the disc.

6. The system defined in claim 5 wherein the means for generating said third signal includes light activated means for reducing the speed of said rollers some time before the roll is fullly wound.

7. The system defined in claim 6 wherein the means for reducing speed comprises a Light Activated Silicon Controlled Rectifier (LASCR) located on said other side of the disc well in from the periphery thereof; a hole in the disc exposing the LASCR to the light from said other source for one angular position of the disc; and a circuit including the LASCR for reducing the reference signal controlling said speed when the LASCR is rendered conductive by momentary exposure to the light passing through said hole.

8. The system defined in claim 4 including means for reducing the speed of said rollers comprising a Light Activated Silicon Controlled Rectifier (LASCR) normally non-conductive and rendered conductive by light directed onto it a little before the roll is fully wound; a relatively high resistance in the circuit of the reference voltage controlling said speed; a circuit connecting the LASCR and a Zener diode in series across said resistance, said Zener diode having a break down voltage appreciably lower than the reference voltage and representing the reduced speed.

9. The system defined in claim 4 characterized in that it has other means following the position of the center of said roll employing light for generating a fourth signal varying in magnitude according to another predetermined program, means for applying said fourth signal to the drive means for said driven rollers for creating a slight difference in speed therebetween varying according to said other program.

References Cited UNITED STATES PATENTS NATHAN L. MINTZ, Primary Examiner 

